When so many resources are available online to learn about a subject, it's hard to understand why someone would pay for a personal trainer.
If you're a complete beginner in the gym, there is so much content online to help you get started and pretty much anything you do will get results. Just by spending a bit of time researching will help you find trustworthy resources to learn from. There are a million videos explaining calories and nutrition too.
If you're already into health and fitness, you shouldn't need the PT as you can learn everything yourself. If you're not into health and fitness, then you're not passionate enough about it, so why get a PT? You don't need any expert knowledge, just go to the gym + eat better and you'll get results.
A PT could be useful for specialised knowledge. If you're trying to achieve a very particular goal in the gym like a certain lift or have some injuries you're trying to recover from, then a PT who has experience with those things makes sense.
Maybe some people don't want to do any research but just want to be healthier without spending too much time on things + they want some motivation and someone to hold them accountable. Also, there is a fair bit of misinformation about this stuff, although I think it's relatively easy to avoid and not do stupid stuff. I think that's a fair reason but imo learning about this stuff online and internalising it very important otherwise it probably won't last very long. And consistency and longevity is essential for success in fitness.